* util/update-grub_lib.in (grub_file_is_not_garbage): New function.
Determines if a file is garbage left by packaging systems, etc.
* util/update-grub.in: Use grub_file_is_not_garbage() as a condition
for processing /etc/grub.d scripts.
* util/grub.d/10_hurd.in: Fix `GRUB_DISTRIBUTOR' comparison.
* util/grub.d/10_linux.in: Likewise. Use grub_file_is_not_garbage()
as a condition for processing Linux images.